1. Field of the Invention
The present invention relates generally to a geartooth sensor and, more particularly, to a geartooth sensor that is disposed in an off axis relationship with the center of rotation of a rotatable member having one or more discontinuities in its surface.
2. Description of the Prior Art
Many different types of geartooth sensors are known to those skilled in the art. One particular type of sensor utilizes a Hall Effect element disposed in a housing which, in many applications, is generally cylindrical with an operative face at one end of the housing. A sensing element, such as a Hall Effect element, is disposed within the housing in association with related circuitry that is connected in electrical communication with the Hall Effect element.
U.S. Pat. No. 4,373,486, which issued to Nichols et al on Feb. 15, 1983, discloses a rotational position and velocity sensor for use with a ferrous disk which is rotatable driven by an internal combustion engine shaft. The disk is provided with inner and outer circular rims, wherein the outer rim has two notches of a predetermined length and position and the inner rim has three notches of predetermined length and position. A permanent magnet is mounted between and radially spaced from two Hall Effect sensor devices in fixed relation to the shaft axis such that the outer rim passes between one Hall Effect device and the magnet and the second rim passes between the second Hall Effect device and the magnet as the shaft is rotated. The notches are positioned to provide four separate two digit binary outputs to a micro processor that provides output signals to ignition coil drivers for the spark ignition devices of the engine and provides spark advancement and coil dwell time variation in accordance with shaft rotational speed and other engine operating conditions.
U.S. Pat. No. 4,524,932, which issue to Bodziak on Jun. 25, 1985, describes a railroad car wheel detector which utilizes a Hall Effect element. The Hall Effect element is incorporated into an integrated circuit with temperature compensation, voltage regulation and amplification. It is mounted on top of a permanent magnet which is made of ceramic material with the critical Hall axis aligned with the magnet pull axis. The permanent magnet and Hall cell assembly is mounted on the rail at a predetermined distance below the top of the rail so that the flange of passing wheels occupies the air gap between the magnet and the rail through which the major portion of the magnetic flux flows. Reduction of the air gap increases the level of the magnet flux and thus the level of voltage output of the Hall cell.
U.S. Pat. No. 4,677,378, which issued to Tokura et al on Jun. 30, 1987, discloses a displacement center which includes a magnetically responsive member and a pair of piezoelectric elements. The sensor is provided with first and second piezoelectric elements, a pair of electrodes at one and the other ends of each of the first and second piezoelectric elements, a permanent magnet which is mechanically fixed to one end of the first piezoelectric element, a small piece which is mechanically fixed to one end of the second piezoelectric element and having approximately identical weight as that of the permanent magnet. It also comprises a base plate to which the other end's first and second piezoelectric elements are mechanically fixed. The preferred embodiment of this invention is illustrated and described in association with a plurality of gearteeth of a rotatable member.
U.S. Pat. No. 4,719,419, which issued to Dawley on Jan. 12, 1988, describes an apparatus that is used for detecting the position of a rotary shaft. It is a non-contact rotary position sensor for measuring rotary position of a shaft and includes an annular magnetic member mounted for rotation with the shaft and a Hall Effect device spaced from the shaft. The annular magnetic membrane includes a magnetic pole pair radially oriented. The magnetic field produced radially outwardly from the annular magnetic member is monopolar. In one embodiment, the annular magnetic member is circular and produces a magnetic field having a uniform magnetic flux density around its entire circumference at equidistant locations extending radially outwardly of the annular magnetic member and the annular magnetic member is mounted eccentric with respect to the shaft.
When a geartooth sensor is associated with a rotatable member, such as a gear, for the purpose of measuring the rotational speed of the rotatable member or, alternatively, its angular position, the Hall Effect element is commonly associated with a biasing magnet that is disposed proximate the Hall Effect element within the housing with the Hall Effect element being disposed between the biasing magnet and the rotatable member. The biasing magnet provides a magnetic field that affects the operation of the Hall Effect element when the proximity of a magnetic material, such as the rotatable member, distorts the magnetic field which is sensed by the Hall Effect element. To facilitate this type of sensing operation, the rotatable member is provided with at least one discontinuity in its surface. In many applications, a single depression is provided in the rotatable member while in other applications a plurality of gearteeth are disposed around the periphery of the rotatable member for sensing by the geartooth sensor.
In a typical application of a geartooth sensor, both the rotatable member and the sensor are disposed within a common apparatus such as an internal combustion engine. The rotatable member can be attached to a camshaft while the geartooth sensor is disposed in an opening within the body of the internal combustion engine. The geartooth sensor is typically disposed in the opening of an engine with the Hall Effect element being locate proximate the surface of the rotatable member.
Because of normal machining tolerances involved in the construction of the engine, or other similar apparatus in which the sensor is to be used, it is difficult to predict the precise distance between the Hall Effect element and the surface of the rotatable member which comprises the one or more discontinuities. Since it is important that the geartooth sensor provide a signal at a precise point of rotation for each discontinuity that it senses, these tolerances can create severe problems during operation of the engine. As will be described in greater detail below, the variation in distance between the Hall Effect element and the surface of the rotatable member can cause the Hall Effect element to sense a discontinuity at different angular positions of the rotatable member, depending on the distance between the Hall Effect element and the surface of the rotatable member. This variation in actuation position of the geartooth sensor can create severe problems since the geartooth sensor is typically used in operations such a the timing of the operation of the engine. It is therefore beneficial to provide a geartooth sensor for these types of applications in which the variability of sensor position caused by the build up of manufacturing tolerances does not adversely affect the angular position of the rotatable member at which the geartooth sensor reacts to the position of a discontinuity in the surface of the rotatable member. This advantage can be provided by a geartooth sensor which is essentially immune to variations in the location of its Hal Effect element relative to the surface of the rotatable member.